A conventional displaying apparatus for displaying an object on a screen typically comprises a line counter, which generates the starting address of each horizontal scanning line. The starting address is sent to a memory counter, wherein the addresses in the frame buffer which correspond, respectively, to the dots, or pixels, constituting the scanning line, are produced. Some display controllers are modified to provide an offset register and a starting address register. These display controllers allow horizontal as well as vertical movements of the displayed images to be effectuated. All these displaying apparatuses are based on the linear manipulation of line addresses corresponding to each scanning line.
Other types of display controllers are also widely used for raster scanning type displays. For example, an image-type display controller has been in TV game equipment for controlling its CRT display. In one of such displays, the CRT is divided into picture (i.e., "image" or "object") elements in an array of 256.times.256 dots. A character consisting of 8.times.8 dots can be displayed in 32.times.32 locations on the screen. A circuit configuration of this type typically comprises a character ROM and a buffer RAM. The buffer RAM has 32.times.32 addresses and one address of the buffer RAM corresponds to a position on the screen of the display. When a character is required to be displayed at a position on the screen, a character number thereof (i.e., corresponding to the character) stored in the character ROM must be stored in one address of the buffer RAM corresponding thereto. During the horizontal scanning on the display, one address of the buffer RAM is selected and one byte at every horizontal scanning is read out from the character ROM as the displayed data.
The image-type display controller described above was modified by Murauchi in U.S. Pat. No. 4,754,270, in which the screen resolution of the CRT was increased to 1024 dots by 256 lines (i.e., the horizontal direction is divided in dots four times as many as the former case). In the display controller disclosed by Murauchi, the character ROM stores the display data of one image in 8.times.8 bits as usual, and the buffer RAM also has a capacity of 32.times.32 addresses as usual and each address thereof corresponds to the position on the screen display. The horizontal address of the buffer RAM is renewed at every 50 nanoseconds and the vertical address thereof is renewed four times in one horizontal blanking period and twice in one vertical blanking period. Start address data for locating the position of the image on the display is loaded into vertical address and horizontal address counters which are then incremented by a carry output from horizontal and vertical adders, respectively. The adders for the vertical and horizontal addresses repeat adding operations according to predetermined addend data. By setting appropriate addend data, the size of the characters and the associated image on a CRT screen can be enlarged or reduced with respect to a normal size.
Prior to the '270 patent of Murauchi, a device called VCO (voltage controlled oscillator) was taught in U.S. Pat. No. 4,107,665, in which the oscillation frequency thereof is changed in response to a data from a CPU and the address counter is incremented synchronously with the oscillation output. The higher the oscillation frequency of the VCO, the smaller the object displayed on the screen. Conversely, the longer the oscillation period of the VCO, the longer the addressing time, and thus the larger the size of the displayed object.
While the enlargement/reduction technique taught by the '270 patent is applicable to a displayed screen image, the voltage controlled oscillator developed in the '655 patent can be applied to the entire screen. However, since the ratio of image enlargement and reduction of the voltage controlled oscillator of the '655 patent is exactly the changing rate of the frequency, and the range of the frequency rate change is limited by the capacitor circuit, the enlargement and reduction ratio cannot be increased to a satisfactory extent. Furthermore, neither of the display control apparatus disclosed in the '270 and '655 can be applied on a scanning line basis.